Electronic systems and circuits have made a significant contribution towards the advancement of modern society and are utilized in a number of applications to achieve advantageous results. Numerous electronic technologies such as digital computers, calculators, audio devices, video equipment, and telephone systems have facilitated increased productivity and reduced costs in analyzing and communicating data, ideas and trends in most areas of business, science, education and entertainment. Electronic systems designed to produce these results often comprise integrated circuits fabricated on semiconductor chips. Transistors are one of the most prolific basic building blocks of modern electronic circuits and they typically require a variety of electrical characteristics to be maintained between different component regions to achieve proper operation. Fabricating relatively small sized high voltage transistors with the ability to maintain requisite electrical relationships between component regions is usually very difficult.
Integrated circuit fabrication usually involves complicated processes that attempt to produce precise components and it is often very difficult to achieve optimized results within requisite tolerances. For example, transistor fabrication processes typically include diffusion and implantation operations directed at creating regions with particular electrical characteristics. Transistors typically include a “negatively” doped region and a “positively” doped region. The operation of the transistor is based upon the electrical interaction of these regions and the ability of these regions to influence current flow under certain conditions. The electrical characteristics of these regions and their “barrier” junctions is critical to proper operation of the device.
Semiconductor integrated circuit (IC) manufacturing efforts are usually complicated by ever increasing demands for reduced component sizes. The desire for smaller transistor sizes is typically driven by several motivations, including a desire for faster devices that provide greater functionality and mobility. Scaling down of IC dimensions typically reduces capacitance which usually permits higher speed performance in integrated circuits. More complicated circuits are usually required to satisfy the demand for greater functionality and there is usually a proportional relationship between the number of components included in an integrated circuit and the functionality, integrated circuits with more components typically provide greater functionality. Including more components within an integrated circuit often requires smaller components densely packed in relatively small areas. Moreover, reducing area of an IC die leads to higher yields in IC fabrication. However, reliably reducing the size of IC components is usually very difficult. For example, as the size of each component region of a transistor is reduced there is typically an adverse impact on the ability of the transistor to maintain critical electrical relationships and characteristics. These adverse impacts are often magnified by the increased field effects created in high voltage transistors.
Reducing the size of a transistor typically involves shorting the channel length. However, shorting channel length can cause several problems. As the channel is shortened, the maximum electric field becomes more isolated near the drain side of the channel causing a saturated condition that increases the maximum energy on the drain side of the device. The high energy often results in the production of “hot” electrons in the channel region that can degrade performance and cause junction breakdown. The problems associated with hot electrons often manifest themselves as undesirable decreases in the saturation current, decreases of the transconductance and a continual reduction in device performance caused by the trapped charge accumulation in the gate. Hot electrons can overcome the potential energy barrier between the silicon substrate and the silicon dioxide layer overlying the substrate, which results in the injection of hot electrons into the gate region. Injection of hot electrons into the gate region can cause alteration of the charge characteristics of the gate region which in turn can detrimentally impact the threshold voltage characteristics (e.g., permanently increase the threshold voltage). The injection of hot electrons can also cause a generation of a undesirable gate current.
Another problem often encountered in short channel junctions is increased punch through effects. Punch through effects are caused by carriers “punching trough” across the channel region from one heavily-doped region to the other in the absence of a gate current (e.g., a current resulting from a voltage being applied to the gate). Punch through typically occurs due to the decreased barrier resistance between a source and drain depletion area that can occur when a channel is shortened. Punch through effects significantly impact the desired ability to control current flow between a source and a drain with a predetermined threshold voltage by producing a current flow regardless of the threshold voltage. Furthermore, the electric fields produced in high voltage applications increase the probability of adverse impacts due to junctions breakdown and punch trough as device sizes and regions are reduced.
Thus, it is desirable for integrated circuit fabrication technologies to provide an advantageous balance between component integrity and increased component density. It is important for source and drain regions to be accurately fabricated to ensure proper operation without defects. It is also desirable for the source and drain formation to be efficient and low cost. Formation of high quality source and drain components that provide desirable operating characteristics can be challenging. Therefore, the ability to precisely form source and drain sections in a convenient and efficient manner is very important.